Legacy satellite communication systems have employed simple “bent-pipe” satellites that relay signals among terminals located in the same large antenna footprint, for example, the continental Unites States. Due to the overlap of transmit and receive coverage areas, separate frequency bands are used for the uplink (to the satellite) and the downlink (from the satellite). The “bent-pipe” designation refers to the fact that the relayed signals are effectively retransmitted after the signals are received by the satellite, as if redirected through a bent pipe. The data in the relayed signals is not demodulated or remodulated as in a “regenerative” or processing satellite architecture; signal manipulation on the satellite in a bent-pipe architecture is generally limited to functions such as frequency translation, filtering, amplification, and the like.
Later satellite communication systems were developed around satellites that employ innovations such as digital channelization and routing of signals, demodulation/routing/remodulation of the data in the relayed signals, narrow antenna footprint “spot” beams to allow frequency reuse, and phased array antennas to allow dynamic placement of coverage areas.
For example, satellites for Mobile Satellite Services (MSS) typically employ spot beam coverage areas with a high degree of frequency reuse. Examples of satellites for MSS include the Inmarsat-4 satellites and the Thuraya satellites. These satellites typically feature a large number of small spot beams covering a large composite area and allow for flexible and configurable allocation of bandwidth. However, the total system bandwidth is very low (such as a 34 MHz allocation at L-band), and service is generally categorized as “narrow band” (e.g., carrier bandwidths of hundreds of kHz), which allows the flexible and configurable bandwidth allocation to be done using digital beamforming techniques. These satellites use a large reflector with an active feed array. The signals from each feed element are digitized, and the beamforming and bandwidth flexibility are provided by a digital signal processor. The digital beamforming is performed on narrowband channels, allowing any narrowband channel on the feeder link to be placed at any frequency for any spot (or other) beam shape.
The Wideband InterNetworking Engineering Test and Demonstration Satellite (WINDS) is an experimental Ka-band satellite system. The satellite implements both fixed spot beams using a fixed multi-beam antenna (MBA) and an active phased array antenna (APAA). The MBA serves fixed beams, and the communications link can be switched over time in a pattern consisting of combinations of receiving and transmitting beams. The APAA has been developed as a beam-hopping antenna with a potential service area that covers almost the entire visible region of earth from the satellite. The APAA can provision communications between arbitrary users using two independently steerable beams for each of the transmitting and receiving antennas. Beam steering is achieved by updating pointing directions via control of digital phase shifters in switching interval slots as short as 2 ms in Satellite Switched Time Division Multiple Access (SS-TDMA) mode, where the shortest beam dwell time corresponds to the slot time of the SS-TDMA system. Beam switching at high speed is supported for up to eight locations per beam. Switching patterns for both the MBA and APAA are uploaded from a network management center.
Spaceway is a Ka-band satellite system that services 112 uplink beams and nearly 800 downlink beams over the United States. The Spaceway satellite uses a regenerative on-board satellite processor to route data packets from one of 112 uplink beams to one of nearly 800 possible downlink beams. At any time the downlink consists of up to 24 hopping beams. The downlink scheduler determines which beams should be transmitting bursts for each downlink timeslot depending on each beams downlink traffic queue and power and interference constraints.
The Wideband Global SATCOM (WGS) satellite, formerly known as the Wideband Gapfiller Satellite, is a U.S. government satellite that employs steerable Ka-band spot beams and X-band beamforming. The Ka-band spot beams are mechanically steered. Up to eight X-band beams are formed by the transmit and receive X-band arrays using programmable amplitude and phase adjustments applied to beamforming modules (BFMs) in each antenna element. Bandwidth assignment is flexible and configurable using a broadband digital channelizer, which is not involved in beamforming.
More recent satellite architectures have resulted in further dramatic increases in system capacity. For example, ViaSat-1 and the Ka-band spot beam satellite architectures disclosed in Dankberg et al. U.S. Pat. App. Pub. No. 2009-0298416, which is incorporated by reference herein in its entirety, can provide over 150 Gbps of physical layer capacity. This spot beam architecture provides over an order of magnitude capacity increase over prior Ka-band satellites. Other satellites, for example KA-SAT and Jupiter, use similar architectures to achieve similarly high capacities. The architecture used in all of these satellites is a “bent pipe” hub-spoke architecture that includes small spot beams targeted at fixed locations. Each spot beam may use a large amount of spectrum, typically 250-1000 MHz. The resulting large capacity is a product of several characteristics of the satellite system, including, for example, (a) the large number of spot beams, typically 60 to 80 or more, (b) the high antenna directivity associated with the spot beams (resulting in, for example, advantageous link budgets), and (c) the relatively large amount of bandwidth used within each spot beam.
The aforementioned high capacity satellite architectures are extremely valuable, but may still be limited in certain respects. For example, scaling the architecture to support higher capacities while maintaining the same spectrum allocation and power budget is typically accomplished using larger reflectors to create spot beams with smaller diameters. The use of smaller diameter spot beams may increase the directivity (or gain) of the satellite antenna, thus enhancing the link signal-to-noise ratio (SNR) and capacity. However, the smaller beams necessarily reduce the coverage area (e.g., the area for which satellite service can be provided). These satellite architectures, therefore, have an inherent tradeoff of capacity versus coverage area.
In addition, these architectures typically place all spot beams, both user beams and gateway (GW) beams, in fixed locations. There is generally no ability to move the spot beams around to accommodate changes in the coverage area. Moreover, the architectures essentially provide uniformly distributed capacity over the coverage area. The capacity per spot beam, for example, is strongly related to the allocated bandwidth per spot beam, which is predetermined for every spot beam and allows for little to no flexibility or configurability.
Although these high capacity architectures are extremely valuable when the desired coverage area is well-known and the demand for capacity is approximately uniformly distributed over the coverage area, the inflexibility of the aforementioned architectures can be limiting for certain applications. What is needed, therefore, is a satellite system architecture that provides high capacity, large coverage areas, increased flexibility, for example, in the locations of the coverage areas and gateways and in the spatial distribution of the capacity, an ability to change coverage areas, gateway locations, and capacity allocation during the lifetime of the satellite, and a flexible design that could be useful in many orbit slots or allow moving the satellite to another orbit slot during the mission lifetime.